1. Field of the Invention
This invention relates to a method of recovering oil from an oil-bearing formation using a liquid containing a surfactant. More particularly, the liquid used to displace oil is a liquid crystal which is especially useful for recovering oil from high brine fields.
2. Description of the Prior Art
The use of microemulsions in secondary and tertiary oil recovery techniques is well-known. Liquid crystal compositions, however, have not been studied to any extent with respect to their potential for the chemically enhanced recovery of crude oil.
An early work which suggests the use of a surfactant system which may possess a liquid crystalline character for oil recovery is U.S. Pat. No. 3,391,736 (Abdo). This patent describes a positive nonsimple fluid useful for recovering oil which possesses a shear thickening response at low shear rates and shear thinning response at high shear rates based on a carboxylate surfactant system. Both of these properties were later claimed in Canadian Pat. No. 921,690 to Murphy who described a system exhibiting birefringence and containing 59-94% water, 3-20% oil, 2-16% surfactant and 1-5% alcohol.
U.S. Pat. No. 3,954,627 (Dreher and Gogarty) describes a composition useful for stimulating oil wells and containing lamellar micelles exhibiting retro-viscous property. These compositions contain from 4-15% surfactant, 30-70% hydrocarbon and 15-66% water, and optionally a co-surfactant and up to 5 wt.% of an electrolyte. They are birefringent and are probably liquid crystalline in nature. Related patents are U.S. Pat. Nos. 3,948,782 and 3,928,215.
More recently, Dreher et al (J. Coll. and Interf. Sci. 57, 379-387 (1976)) studied a system composed of alkylbenzene sulfonate, hydrocarbon, water and co-surfactant as a model for a tertiary oil recovery fluid. The rheological properties indicated that this system could exist as either a microemulsion or liquid crystal depending on the particular composition. Further studies on certain alkylaryl and petroleum sulfonates indicate that liquid crystalline phases can be formed and the formation of these phases are of interest for chemically enhanced oil recovery since they can effect, for example, interfacial tension, viscosity, and retention. For reference, see articles in the Preprints, Div. Petr. Chem., A.C.S., 23(2), 644 (1978) by Scriven et al and ibid., 23(2), 682 (1978) by Miller et al.
Finally, Shah et al in the J. Amer. Oil Chem. Soc., 55, 367 (1978) disclosed a surfactant system based on a mixture of petroleum sulfonate and ethoxylated alkane sulfonate in brine which was tolerant to salts including large amounts of CaCl.sub.2 and MgCl.sub.2. At certain salt concentrations, it was reported that a stable birefringent phase formed.
In spite of the hypotheses of the above articles, it is generally believed in the art that liquid crystals are not a practical fluid for oil displacement. The reason for this is the high viscosity associated with liquid crystals. For example, Reed and Healy in an article entitled, "Some Physiochemical Aspects of Microemulsion Flooding: A Review," which appeared in the monograph Improved Oil Recovery by Surfactant and Polymer Flooding, Shah and Schechter, Academic Press, N.Y., 1977, emphasized in part D, especially Page 402, that the excessive viscosity associated with lamellar structures would prohibit application of these compositions to tertiary oil recovery. This pessimism is reflected in the absence of published experiments wherein liquid crystals were used to displace oil. In addition to viscosity considerations, high brine concentrations provide an additional negative factor since it is generally believed that high salt concentrations destabilize liquid crystals.
It would be highly desirable to show that lyotropic liquid crystals containing minor amounts of surfactants and oil are stable in the presence of highly concentrated brine and provide an alternative and improved means to displace oil as compared to microemulsions.